System and method for magnetic-resonance-guided electrophysiologic and ablation procedures

ABSTRACT

A system and method for using magnetic resonance imaging to increase the accuracy of electrophysiologic procedures includes an invasive combined electrophysiology and imaging antenna catheter which includes an RF antenna for receiving magnetic resonance signals and diagnostic electrodes for receiving electrical potentials. The combined electrophysiology and imaging antenna catheter is used in combination with a magnetic resonance imaging scanner to guide and provide visualization during electrophysiologic diagnostic or therapeutic procedures, such as ablation of cardiac arrhythmias. The combined electrophysiology and imaging antenna catheter may further include an ablation tip, and be used as an intracardiac device to deliver energy to selected areas of tissue and visualize the resulting ablation lesions.

RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/314,241 filed Dec. 22, 2005 which is a continuation of U.S. patentapplication Ser. No. 10/424,093, filed Apr. 28, 2003, which is adivisional application of U.S. patent application Ser. No. 09/428,990(U.S. Pat. No. 6,701,176) filed Oct. 29, 1999, and claims the benefit ofU.S. Provisional Patent Application No. 60/106,965 filed Nov. 4, 1998,the entire contents of all of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates in general to ablation and electrophysiologicdiagnostic and therapeutic procedures, and in particular to systems andmethods for guiding and providing visualization during such procedures.

2. Related Art

Atrial fibrillation and ventricular tachyarrhythmias occurring inpatients with structurally abnormal hearts are of great concern incontemporary cardiology. They represent the most frequently encounteredtachycardias, account for the most morbidity and mortality, and, despitemuch progress, remain therapeutic challenges.

Atrial fibrillation affects a larger population than ventriculartachyarrhythmias, with a prevalence of approximately 0.5% in patients50-59 years old, incrementing to 8.8% in patents in their 80's.Framingham data indicate that the age-adjusted prevalence has increasedsubstantially over the last 30 years, with over-2 million people in theUnited States affected. Atrial fibrillation usually accompaniesdisorders such as coronary heart disease, cardiomyopathies, and thepostoperative state, but occurs in the absence of any recognizedabnormality in 10% of cases. Although it may not carry the inherentlethality of a ventricular tachyarrhythmia, it does have a mortalitytwice that of control subjects. Symptoms which occur during atrialfibrillation result from the often rapid irregular heart rate and theloss of atrio-ventricular (AV) synchrony. These symptoms, side effectsof drugs, and most importantly, thromboembolic complications in thebrain (leading to approximately 75,000 strokes per year), make atrialfibrillation a formidable challenge.

Two strategies have been used for medically managing patients withatrial fibrillations. The first involves rate control andanticoagulation, and the second involves attempts to restore andmaintain sinus rhythm. The optimal approach is uncertain. In themajority of patients, attempts are made to restore sinus rhythm withelectrical or pharmacologic cardioversion. Current data suggestanticoagulation is needed for 3 to 4 weeks prior to and 2 to 4 weeksfollowing cardioversion to prevent embolization associated with thecardioversion. It remains controversial whether chronic antiarrhythmictherapy should be used once sinus rhythm is restored. Overall,pharmacologic, therapy is successful in maintaining sinus rhythm in 30to 50% of patients over one to two years of follow-up. A majordisadvantage of antiarrhythmic therapy is the induction of sustained,and sometimes lethal, arrhythmias (proarrhythmia) in up to 10% ofpatients.

If sinus rhythm cannot be maintained, several approaches are used tocontrol the ventricular response to atrial fibrillation. Pharmacologicagents which slow conduction through the AV node are first tried. Whenpharmacologic approaches to rate control fail, or result in significantside effects, ablation of the AV node, and placement of a permanentpacemaker is sometimes considered. The substantial incidence ofthromboembolic strokes makes chronic anticoagulation important, butbleeding complications are not unusual, and anticoagulation cannot beused in all patients.

Medical management of atrial fibrillation, therefore, is inadequate.

In addition to medical management approaches, surgical therapy of atrialfibrillation has also been performed. The surgical-maze procedure,developed by Cox, is an approach for suppressing atrial fibrillationwhile maintaining atrial functions.

This procedure involves creating multiple linear incisions in the leftand night atria.

These surgical incisions create lines of conduction block whichcompartmentalize the atrium into distinct segments that remain incommunication with the sinus node. By reducing the mass of atrial tissuein each segment, a sufficient mass of atrial tissue no longer exists tosustain the multiple reentrant rotors, which are the basis for atrialfibrillation. Surgical approaches to the treatment of atrialfibrillation result in an efficacy of >95% and a low incidence ofcomplications. Despite these encouraging results, this procedure has notgained widespread acceptance because of the long duration of recoveryand risks associated with cardiac surgery.

Invasive studies of the electrical activities of the heart(electrophysiologic studies) have also been used in the diagnosis andtherapy of arrhythmias, and many arrhythmias can be cured by selectivedestruction of critical electrical pathways with radio-frequency (RF)catheter ablation. Recently, electrophysiologists have attempted toreplicate the maze procedure using radio-frequency catheter ablation,where healing destroys myocardium. The procedure is arduous, requiringgeneral anesthesia and procedure durations often greater than 12 hours,with exposure to x-rays for over 2 hours. Some patients have sustainedcerebrovascular accidents.

One of the main limitations of the procedure is the difficultyassociated with creating and confirming the presence of continuouslinear lesions in the atrium. If the linear lesions have gaps, thenactivation can pass through the gap and complete a reentrant circuit,thereby sustaining atrial fibrillation or flutter. This difficultycontributes significantly to the long procedure durations discussedabove.

Creating and confirming continuous linear lesions could be facilitatedby improved techniques for imaging lesions created in the atria. Such animaging technique may allow the procedure to be based purely on anatomicfindings.

The major technology for guiding placement of a catheter is x-rayfluoroscopy. For electrophysiologic studies and ablation, frame rates of7-15/sec are generally used which allows an operator to seex-ray-derived shadows of the catheters inside the body. Since x-raystraverse the body from one side to the other, all of the structures thatare traversed by the x-ray beam contribute to the image. The image,therefore is a superposition of shadows from the entire thickness of thebody. Using one projection, therefore, it is only possible to know theposition of the catheter perpendicular to the direction of the beam. Inorder to gain information about the position of the catheter parallel tothe beam, it is necessary to use a second beam that is offset at someangle from the original beam, or to move the original beam to anotherangular position. Since x-ray shadows are the superposition ofcontributions from many structures, and since the discrimination ofdifferent soft tissues is not great, it is often very difficult todetermine exactly where the catheter is within the heart. In addition,the boarders of the heart are generally not accurately defined, so it isgenerally not possible to know if the catheter has penetrated the wallof the heart.

Intracardiac ultrasound has been used to overcome deficiencies inidentifying soft tissue structures. With ultrasound it is possible todetermine exactly where the walls of the heart are with respect to acatheter and the ultrasound probe, but the ultrasound probe is mobile,so there can be doubt where the absolute position of the probe is withrespect to the heart. Neither x-ray fluoroscopy nor intracardiacultrasound have the ability to accurately and reproducibly identifyareas of the heart that have been ablated.

A system known as “non-fluoroscopic electroanatomic mapping (Ben-haim;U.S. Pat. No. 5,391,199), was developed to allow more accuratepositioning of catheters within the heart. That system uses weakmagnetic fields and a calibrated magnetic field detector to track thelocation of a catheter in 3-space. The system can mark the position of acatheter, but the system relies on having the heart not moving withrespect to a marker on the body. The system does not obviate the needfor initial placement using x-ray fluoroscopy, and cannot directly imageablated tissue.

MR is a known imaging technique which uses high-strength magnetic andelectric fields to image the body. A strong static magnetic field(between the magnet poles in this example) orients the magnetic momentsof the hydrogen nuclei. RF time-varying magnetic field pulses change thespatial orientation of the magnetic moments of the nuclei. To exert asignificant torque on the moment, the frequency of the magnetic fieldmust be equal to the frequency of precession of the magnetic moment ofthe nuclei about the direction of the static magnetic field. Thisfrequency of precession is a natural, or resonance, frequency of thesystem (hence Magnetic Resonance Imaging).

The time-varying gradient magnetic field is used for spatial encoding ofthe signals from the issue. The magnitude of the gradient field is alinear function of the space coordinates in the magnet. As a result ofthe addition of the static and gradient magnetic fields, the total localmagnetic field and, thus, the local resonance frequency, becomes alinear function of position. Thus, imaging tissues in any plane can beaccomplished because the location of each volume element is known inthree-dimensional space.

MRI is generally considered a safe technique, since no x-rays are usedand the electromagnetic fields do not, by themselves, cause tissuedamage.

While MRI may provide the visual guidance necessary for creating andconfirming linear lesions, it has been assumed that electrical wiresimplanted in a patient can act as antennas to pick up radio-frequencyenergy in an MR system and conduct that energy to the patient, therebycausing tissue injury.

Magnetic resonance imaging has been used to guide procedures in which RFenergy is applied to non-contractile organs such as the brain, liver andkidneys to ablate tumors. However, these systems are not suitable foruse in the heart.

U.S. Pat. No. 5,323,778 to Kandarpa et al. discloses a method andapparatus for magnetic resonance imaging and tissue heating. There is noprovision in the disclosed probe for measuring electrical signals; and,it is unclear how much resolution the probe provides.

OBJECTS AND SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide an improved systemand method for guiding and/or providing visualization duringelectrophysiologic procedures.

It is a further object of the invention to provide a system and methodfor guiding or visualizing ablation procedures which is suitable for usein the heart and other structures.

It is a further object of the invention to provide a system and methodfor imaging ablation lesions with increased resolution and reliability.

The invention provides a system and method for using magnetic resonanceimaging to increase the safety and accuracy of electrophysiologicprocedures. The system in its preferred embodiment provides an invasivecombined electrophysiology and imaging antenna catheter which includesan RF antenna for receiving magnetic resonance signals and diagnosticelectrodes for receiving electrical potentials. The combinedelectrophysiology and imaging antenna catheter is used in combinationwith a magnetic resonance imaging scanner to guide and providevisualization during electrophysiologic diagnostic or therapeuticprocedures. The invention is particularly applicable to catheterablation of atrial and ventricular arrhythmias. In embodiments which areuseful for catheter ablation, the combined electrophysiology and imagingantenna catheter may further include an ablation tip, and suchembodiment may be used as an intracardiac device to both deliver energyto selected areas of tissue and visualize the resulting ablationlesions, thereby greatly simplifying production of continuous linearlesions. Additionally, the ablation electrode can be used as an activetracking device that receives signal from the body coil excitation.Gradient echoes are then generated along three orthogonal axes tofrequency encode the location of the coil and thus provide thethree-dimensional space coordinates of the electrode tip. These numericcoordinates can then be used to control the imaging plane of thescanner, thereby allowing accurate imaging slices to be automaticallyprescribed though the anatomic target for RF therapy. The inventionfurther includes embodiments useful for guiding electrophysiologicdiagnostic and therapeutic procedures other than ablation. Imaging ofablation lesions may be further enhanced by use of MR contrast agents.The antenna utilized in the combined electrophysiology and imagingcatheter for receiving MR signals is preferably of the coaxial or“loopless” type that utilizes a helical whip.

High-resolution images from the antenna may be combined withlow-resolution images from surface coils of the MR scanner to produce acomposite image. The invention further provides a system for eliminatingthe pickup of RF energy in which intracardiac wires are detuned, by forexample low-pass filters, so that they become very inefficient antennas.An RF filtering system is provided for suppressing the MR imaging signalwhile not attenuating the RF ablative current. Steering means may beprovided for steering the invasive catheter under MR guidance. Lastly,the invention provides a method and system for acquisition ofhigh-density electroanatomic data using a specially designedmulti-electrode catheter and the MRI scanner. This will be achieved byusing an active tracking system that allows the location of eachelectrode to be determined.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments as illustrated in the accompanyingdrawings, in which reference characters refer to the same partsthroughout the various views. The drawings are not necessarily to scale,emphasis instead being placed upon illustrating principles of theinvention.

FIG. 1 shows a schematic view of a combined electrophysiology andimaging antenna catheter in accordance with a preferred embodiment ofthe invention.

FIG. 2 shows a cross-sectional detail view of a tip portion of combinedelectrophysiology and imaging antenna catheter in accordance with apreferred embodiment of the invention.

FIG. 3 shows a block diagram illustrating the operation of an MRIscanner system which may be used in connection with the system andmethod of the invention.

FIG. 4 illustrates a schematic block diagram showing an example ofradio-frequency filters which may be used in accordance with theinvention.

FIG. 5 shows a graphic representation of electrical signals measuredfrom a catheter in accordance with the invention during MR imaging.

FIG. 6 shows a high-level block diagram illustrating an ablation systemincorporating radio-frequency filters in accordance with a preferredembodiment of the invention.

FIG. 7 shows three-dimensional reconstructions of MR images from planarsections.

DETAILED DESCRIPTION

The invention in its preferred embodiment uses MR imaging to allowcatheters to be placed without radiation, and provides very accuratelocalization of catheter tips in 3-dimensional space. With current MRIscanners, resolution is limited by the distance the RF coil is from thevolume of tissue being imaged. RF from any particular imaging volume ispicked up by the surface coil. The gradients select a volume inside thebody for imaging, but the coil outside the body picks up the signal fromthe volume. The farther the surface coil is from the imaging volume, themore noise will be present.

In accordance with a preferred embodiment of the invention, anintracardiac receiving coil/antenna is used so that the receivingcoil/antenna is closer to the imaging volume (lesions), thereby reducingnoise, increasing signal, and improving resolution where it is neededmost.

In a first embodiment of the invention, MRI is used to facilitatecatheter ablation of atrial fibrillation by guiding creation ofcontinuous linear ablation lesions and confirming that a complete linearlesion has been created (line of block). The visualization of areas ofablation may allow a reduction in the number of lesions needed, and mayalso reduce the number of recurrences, by more accurately ablating thearrhythmias.

FIGS. 1 and 2 show schematic and detail views, respectively, of acombined electrophysiology and imaging antenna catheter in accordancewith a preferred embodiment of the invention. The device of theinvention is used in combination with an MRI scanner such that RF energycan be delivered to selected areas of tissue, the tissue imaged with aninvasive (e.g., intracardiac) antenna, and RF lesions or other targetscan be visualized in both high and low resolution modes. MRI allowsvisualization of lesions in the ventricle with the use of surface coils,and in the atria with surface coils and/or the intracardiaccatheter-antenna. With these catheter antennae, the image can be alignedperpendicular to the catheter, such that the best resolution will be atsite of the lesion. This lesion visualization can be used for (1)precise titration of therapy, (2) the ability to test the length anddepth of lesions from new ablation-energy sources, and (3) accurateassessment of the success of making lines of ablation.

In addition to catheter-antenna, high-resolution imaging can also bedone with receivers that contain loops that are placed inside the body.These loops may be fixed in size or may be expandable once placed in thebody to increase their surface area.

MRI can also be used in accordance with the invention to guide otherprocedures.

In cardiology, accurate anatomic information, combined with electricalmeasurements, allows improved study of the pathophysiology ofarrhythmias, stunning, remodeling, and tachycardia-induced myopathy.Outside of cardiology, it has already been demonstrated that biopsies ofliver, kidney, adrenal gland, neck masses, and lymph nodes could all bedone safely and accurately with MR-guidance. With extensions of thebiopsy technique, MRI-guided ablation of tumors such as metastatic liverdisease, brain tumors, and prostate cancer, may allow treatment withless morbidity and less cost than conventional open surgery.

FIG. 1 shows a schematic diagram of the device 1 of the invention andFIG. 2 shows a detail view of a tip portion 15 of the device. The systemof the invention preferably comprises a combined electrophysiology andimaging antenna catheter 1 which is used in conjunction with an MRIscanner such that visualization can be performed simultaneously withdelivery of RF energy to selected areas of tissue for ablation. Inembodiments designed for cardiac ablation applications, the length ofthe invasive portion of the device is preferably at least 1200millimeters long so that the tip can be placed into the heart from thefemoral artery or vein. The diameter of the device is approximately 2.5mm.

The device preferably includes between one and three diagnosticelectrodes 11 for receiving electrical potentials, e.g., intracardiacpotentials, in connection with electrophysiological procedures andtesting. In embodiments useful for ablation applications, the devicefurther includes an ablation tip 13. The electrodes 11 are preferablyfabricated from platinum or gold. The tip portion 15 of the device isdeflectable by a steering wire 5, preferably of titanium construction,that is inside a low-friction sheath, preferably of Teflon construction.The steering wire 5 connects to a steering knob 7 and moves toward oraway from the tip when the steering knob 7 is rotated, deflecting thetip in the appropriate direction. A connector 9 is used to interconnectthe antenna 3 with receiver or scanner circuitry, which is discussed infurther detail below, and is also used to connect the electrodes 11 toexternal electronic devices.

The device of the invention includes an antenna portion 19, which may beof various suitable designs. In the preferred embodiment, a flexible,helical whip coaxial loopless antenna is used. Such an antenna can bemade by removing a section of the shield from an antenna coaxial cable,so as to form a ‘whip’ with the center conductor.

To avoid direct biofluid contact with conductive components of thecatheter it will be covered with a non-conductive dielectric material.Addition of insulation to the antenna, however, increases the whiplength required for optimal image quality to a length that prohibitivelylarge for in vivo use. Incorporating a helical whip in the looplessantenna design overcomes this limitation by allowing up to 10 times theelectrical length to be achieved in the same physical length as astraight conductor whip. In addition to these electromagneticadvantages, the helical antenna whip also improves the mechanicalproperties of the device and thereby greatly improve intravascular andintracardiac navigation of the catheter without kinking, folding ormechanical failure of the whip. The flexible helical whip has guidewireproperties and thus reduces the risks of vascular or cardiacperforation. The length of helical whip can be varied to help in tuningthe antenna to the optimal impedance and in optimizing thesignal-to-noise ratio. Further details regarding the structure anddesign of suitable loopless antennas can be found in U.S. Pat. No.5,928,145, issued Jul. 27, 1999, the entire disclosure of which isincorporated herein by reference.

Since loops can receive more signal in a given imaging volume, anantenna incorporating a loop may provide an improved signal-to-noiseratio, resulting in clearer images. A loop can be formed, where theantenna whip 21 is connected to the antenna body 19 via a miniaturecapacitor. A balloon can be incorporated into the catheter, and the loopcan be attached to the surface of the balloon. When the balloon isinflated, the loop will expand.

In embodiments of the invention wherein a coaxial loopless antenna isutilized, a helical whip portion 21 of the flexible antenna protrudesfrom the distal tip to complete the dipole antenna. The whip portion 21is coated with an insulating layer and its tip 23 can be exposed andformed into a “J” to help prevent the whip from perforating internalphysiological structures. The antenna whip portion 21 should beinsulated from the ablation tip.

When the device of the invention is used for intracardiac ablationprocedures, tissue is imaged with the antenna and RF lesions can bevisualized in both high and low resolution modes. As is discussed indetail below, the images may be enhanced with MRI contrast, such asgadolinium. Software can be provided for optimally visualizing thelesions, and for allowing the operator to change viewing perspective innear-real time.

As is set forth above, embodiments of the invention which are useful forablation procedures preferably include an ablation tip 13. As analternative to the preferred embodiment wherein the active element ofthe antenna runs within the catheter in a coaxial fashion, the RFablation element in the ablation tip may be designed to serve both as anRF ablation transmitter and as a receiver coil for MR imaging. In suchembodiments, a switching device can be used to switch the catheterbetween imaging and ablation modes. When not in ablation mode, theablation electrode, and the other electrodes on the catheter, can beused to measure electrical signals.

Another embodiment of the combined antenna and RF probe device is theuse of untuned RF electrodes as tracking devices. Single or multiple RFelectrodes may serve as small RF coils that receive signal from the bodycoil excitation and then are frequency encoded in three orthogonalplanes. These three space numeric coordinates can then be used toautomatically control the imaging plane of the scanner, allowing optimalimaging of the target region for RF therapy. Additionally, as theelectrodes can also acquire bioelectric signals, electrode location dataallows the generation of true electroanatomic data.

For most applications, the impedance of the imaging antenna must matchthe impedance of the input amplifier. With an ordinary 64 MHz inputamplifier, this impedance is 50 Ohms. A number of matching networks arepossible, the simplest being a series capacitor of an appropriate value.A network analyzer can be used to allow optimal matching of differentantenna designs. o customize matching to an individual patient, thenetwork analyzer can be automated and incorporated into the matchingnetwork to automatically tune the matching network after the antenna hasbeen placed into the patient.

The catheter antenna device of the invention in accordance with itspreferred embodiment is constructed so as to be fully MRI-compatible.Specifically, it's design and materials are selected such that (1) theimage is not significantly distorted by the device; (2) the MRIelectromagnetic fields do not alter the normal functioning of thedevice; (3) cardiac arrhythmias are not produced by the device, and (4)no damage to the tissue is produced by radio-frequency energy receivedfrom the MRI scanner. The presence of even small amounts of magneticmaterial in the imaging fields can produce substantial amounts of imagedistortion. This distortion is caused by perturbation of the imagingmagnetic field. The most distortion is caused by ferromagnetic materials(iron, nickel, cobalt). Little if any distortion is produced bymaterials that do not become significantly magnetized (low magneticsusceptibility) by the MRI magnetic field. Metals which do not producesignificant magnetization include copper, gold, platinum and aluminum.Many plastics and synthetic fibers are entirely non-magnetic and do notdistort the images.

FIG. 3 shows a block diagram illustrating the operation of an MRIscanner system which may be used in connection with the system andmethod of the invention.

A magnet is provided for creating the magnetic field necessary forinducing magnetic resonance. Within the magnet are gradient coils forproducing a gradient in the static magnetic field in three orthogonaldirections. Within the gradient coils is an RF coil.

The RF coil produces the magnetic field necessary to rotate the spins ofthe protons by 90° or 180°. The RF coil also detects the signal from thespins within the body. A computer is provided for controlling allcomponents in the imager. The RF components under control of thecomputer are the RF frequency source and pulse programmer. The sourceproduces a sine wave of the desired frequency. The pulse programmershapes the RF pulses, and the RF amplifier increases the pulse power upto the kilo-watt range. The computer also controls the gradient pulseprogrammer which sets the shape and amplitude of each of the threegradient fields. The gradient amplifier increases the power of thegradient pulses to a level sufficient to drive the gradient coils.

The invention in accordance with a preferred embodiment further includesfilter means and shielding for protecting electronic equipment (e. g.,the MR scanner) from RF produced by the ablation system, for protectingthe ablation and measuring system from RF produced by the MR scanner,and for allowing measurement of the relevant electrical signals. Withoutadequate radio-frequency filters, the electronics attached to thecatheter may malfunction during imaging. FIG. 4 illustrates a schematicblock diagram showing an example of radio-frequency filters which may beused in accordance with the invention. Low-pass filters using 1-henryinductors made without magnetic materials, and 220 picofarad capacitors,have optimal attenuation of the 64 Mhz radio-frequency energy present inthe 1.5 Tesla MR scanner. A number of filter topologies were tested, andthe two stage filter shown in FIG. 4 had the best results. A separatetwo-stage filter (Li, L3, Cl, C3; and L2, L4, C2, C4), is preferablyplaced in each wire to the catheter. These filters can reduce the 15-32volts of radio-frequency pickup down to a few millivolts and cause noproblems with the electronics.

The output of the RF filters can be applied to a series of activefilters. The active filters may comprise, e. g., a sixth order,Chebyshev (1 dB ripple), low-pass filter (50-300 Hz corner); then asecond order, Chebyshev (1 dB ripple), high-pass filter (3-50 Hzcorner); and then a 60 Hz notch filter. These filters limit the signalbandwidth, and substantially reduce gradient-field-induced noise—seeFIG. 5(c), discussed below. The gradient field noise was not rejected bythe RF filters. This filter arrangement is used in thecatheter-intracardiac electrogram measuring circuit. The circuit forablation does not incorporate the active filters, since while the RFfiltering system is designed to suppress the 64 MHz imaging signal. Itdoes not attenuate the RF ablative current, since the radio frequency ofthe ablation system is 200-800 kHz, and the corner for the low-pass RFfilters is 1-10 MHz. The ablation circuit does not need thelower-frequency filters, since that circuit is not being used to measureelectrograms.

FIG. 5 shows a graphic representation of electrical signals measuredfrom a catheter in accordance with the invention during MR imaging. FIG.5(a) shows the signals measured from a catheter without the use of RFfilters; it can be seen that the ECG is obscured by noise (32 voltspeak-to-peak). FIG. 5(b) shows such signals wherein RF filters are used;it can be seen that nearly all radio-frequency interference is removedand an ECG signal is now apparent. The pairs of vertical lines areartifacts from the gradient fields. FIG. 5(c) shows such signals whereinactive RF filters are used; it can be seen that most of the gradientartifact is also suppressed.

FIG. 6 shows a high-level block diagram illustrating an ablation systemincorporating the filters described above. The RF Generator maycomprise, e.g., a standard clinically approved ablation unit, such asthose commercially available from Medtronic, having an RF outputfrequency of 482.65 kHz and an output of 50 W into a 50-250 Q load. Theoutput frequency from the RF generator is directed to the ablationcatheter through two filter assemblies (low pass, 2 Mhz corner). Bothfilter assemblies are fully shielded and are connected by fully shieldedcable. The ECG amplifiers incorporate the active filters as describedabove. The dispersive ground electrode consists of a largeconductive-adhesive pad that is attached to the skin of the animal tocomplete the circuit. The defibrillator (identified as “defib” in FIG.8) may comprise a standard defibrillator used in ablation procedures.

It is important that the location of the tip of the catheter can beaccurately determined. A number of modes of localization can be used.Because the catheter is a receiver it can be used to directly image thetissue around it. This image can be viewed on its own at highresolution, or, it can be viewed at low resolution as an overlay on alarge field-of-view “scout” image obtained with an auxiliary coiloutside the body. The location of the catheter in the body can betracked by the bright line of signal moving in the scout image. Thescout image can be updated at an interval set by the user to compensatefor patient motion. An interactive control will allow the physician to“zoom in” towards the bright catheter, finally resulting in a highresolution image around the catheter tip. The “zoom” function can beachieved with interactive control of the imaging gradients.

A composite “medium resolution” resolution image can be used toconstruct a three-dimensional map of the areas in the heart that haveundergone ablation. These areas will be marked by elevated T2 values, ordecreased T1 values during Gd infusion.

A composite three-dimensional rendering of the heart can be updatedafter each ablation and displayed with an appropriate renderingtechnique.

The guidance of the catheter tip to the next site of ablation, or tofill in a previous ablation line can be assisted using the MR images.This assistance can be entirely passive, in that the physician uses theimages to manipulate the catheter, or automatic tracking and feedbackcould assist that physician to steer the catheter.

The lesions may be visualized using standard imaging techniques. It maybe necessary to MR contrast to enhance the lesions to allow adequatevisualization to occur. One such enhancement method usesgadolinium-DTPA, but other suitable contrast agent could be used. Therationale underlying the utilization of gadolinium-DTPA based contrastagents to enhance signal intensity in atrial or ventricular myocardiuminjured by RF during therapeutic ablation is based on the followingobservations: 1) Gadolinium-DTPA exerts its signal enhancing effect byinteracting with water protons and inducing a shorter relaxation time inresponse to any given radio-frequency stimulus. This effect creates theimage contrast necessary to allow distinction in relation to regionsunaffected by contrast. 2) Gadolinium-DTPA is a large molecule whichcannot penetrate the uninjured cell membrane and is therefore restrictedto the extracellular space in uninjured myocardium. After the RF burn,the injured membrane allows penetration of the contrast agent thusincreasing significantly the volume of distribution for the contrastagent and resulting in a ‘brighter’ voxel of tissue on TI weightedimages. 3) This difference in voxel content of water protons potentiallyexposed to the gadolinium-DTPA molecule creates the possibility ofdistinguishing injured from non-injured tissue with greater spatialresolution than in non-enhanced images.

Gadolinium-DTPA can be injected prior to the RF ablation protocol toenhance injured myocardium as the lesions are produced. The agent takes5-10 minutes to equilibrate between extracellular and intracellularspaces and a few hours to be eliminated through the kidneys. The agentis routinely used in brain MRI studies to highlight areas ofinflammation and in cardiac MR studies to delineate myocardial regionsinjured by prolonged ischemia. Gadolinium-DTPA has an appropriate safetyprofile and except for occasional nausea, does not cause side effectsleading to discomfort or complications in patients.

Imaging of ablated lesions may be further enhanced by use of thermalimaging techniques. Thermal imaging can be accomplished by using phasedifferences in MR signals.

Three-dimensional image reconstruction can be performed using the systemand method of the invention. FIG. 7 shows three-dimensionalreconstructions of MR images from planar sections. In particular, FIG. 7shows three-dimensional reconstructions of images during activation ofthe left ventricle from a right ventricular pacing site. In FIG. 7, thewhite areas show the spread of mechanical activation as the wave ofelectrical activation spreads across the left ventricle from the rightventricular pacing site. Similar image processing techniques can be usedfor visualizing ablated areas.

The advantages of the system and method for MR-guided electrophysiologyin accordance with the invention will now be discussed in furtherdetail.

Recent advances in MRI technology enable frame rates higher than 10/sec.This exceeds the frame rate often used in current pulsed x-rayfluoroscopy systems. When the depth dimension of the MRI slice is set aslarge as the body depth, the resulting 2-dimensional image sequence canserve as an effective substitute for x-ray fluoroscopy.

The system can thus facilitate catheter placement for EP study withreal-time imaging, without the need for ionizing radiation. Cathetersused in this system must be composed entirely of non-ferromagneticmaterials, so as not to perturb the electromagnetic gradient fieldrequired for distortion-free MR imaging.

MRI allows for precise localization of object elements inthree-dimensional space. Catheter tip position within the heart can thusbe determined accurately and precisely, and can then be displayedsuperimposed on anatomically accurate reconstructions of cardiacarchitecture. This functionality is not possible with x-ray fluoroscopy.

Electrical activation timing information obtained via an EP mappingcatheter, when combined with catheter localization information, enablesaccurate color-coded activation maps. This capability is most useful indetermining the site of origin of an atrial or ventricular tachycardia.

Activation maps can be superimposed on anatomically accuratereconstructions of cardiac structure. Spatially accurate voltage data,however, requires knowledge of the location of each electrode incontract with the myocardium. This can be achieved by using high-densitybasket catheter electrodes in conjunction with active tracking RF coils.Each untuned electrode is capable of receiving signal, which in turn,provides the 3-space coordinates of each electrode. Electrical dataoriginating from each known electrode position allows generation ofactivation and voltage maps on true anatomic structures.

This provides significant advantages beyond the capabilities of thenon-fluoroscopic electroanatomic mapping system noted above, since thatsystem does not provide accurate anatomic information, again withoutadditional hardware.

An imaging antenna can be incorporated into a steerable mapping/ablationcatheter, enabling high-resolution imaging in the region near thecatheter tip. The image obtained with this antenna has a similar radiusof view as that with intracardiac ultrasound, but with far greaterresolution. Furthermore, this high-resolution image is obtained withoutthe need for placement of an additional catheter, as is required withintracardiac ultrasound.

High-resolution images derived from the internal antenna can be combinedwith lower-resolution wide-field images obtained with the external coilinto a single image.

This composite image will display the entire cardiac cross section withenhanced resolution in the area of greatest interest When theablation/imaging catheter is used for the delivery of ablativeradio-frequency energy, the high-resolution image obtained via thiscatheter enables visualization of the lesion and of lesion growth. Itmay also be possible to visualize lesions with surface coils alone, ifthe tissue is thick enough.

Directional orientation, as well as location, of the catheter tip can bedetermined in three-dimensional space. The high-resolution image dataobtained via the internal antenna can be displayed in any plane, and inparticular, in the plane orthogonal to the catheter. Since the image isobtained with the same catheter that is delivering the ablative energy,the orthogonal-plane image is guaranteed to display the lesion at itsmaximal radius, without the need to manipulate a second (imaging)catheter into alignment with the ablation catheter. Lesion size willthus not be underestimated as often occurs with intracardiac ultrasound.In the latter case, the imaging catheter differs from the ablationcatheter. It is therefore not necessarily imaging at the same level asthe ablation catheter tip, and is not necessarily parallel to theablation catheter so the image plane is oblique to the lesion equator.

MR is an imaging modality that can be tuned to characterize tissuephysiology as well as structure. This enables imaging of lesions byvirtue of changes in structure and cell function that occur withfulguration. Injection of gadolinium further enhances the MR imagecontrast between healthy and ablated myocardium. Intracardiacultrasound, on the other hand, enables visualization of lesions only tothe extent that tissue echogenicity is altered.

Because the MRI-guided EP system of the invention combinestwo-dimensional real-time image sequences, accurate three-dimensionalcatheter tip localization for activation mapping, and the ability to seemyocardial tissue and lesion growth, it offers the best features ofx-ray fluoroscopy, the non-fluoroscopic electroanatomic mapping system,and intracardiac ultrasound all at once without ionizing radiation,extra venipunctures, or excessively expensive catheters.

High-resolution visualization of ablative lesions by the internal MRantenna allows for documentation of whether or not RF applicationresulted in successful lesion development and of where lesions have andhave not yet been made. This facilitates efficient catheter placement sothat RF is applied only to tissue not previously ablated.

The high-resolution images obtained with the internal MR antenna enablesvisualization of the relatively thin atrial wall. This structure may notbe well visualized by the external MR coil due to lack of adequateresolution. If the atrial wall or other anatomical structures to bevisualized have thick enough walls, which does occur, adequatevisualization may be obtained with surface coils alone.

The combination of the high-resolution visualization and imagesdiscussed above makes high-resolution MRI guidance ideal forvisualization and verification of ablative lesion lines, particularly inatrial tissue. This is useful for ablation of the reentrant circuit intypical atrial flutter and is crucial for successful ablation of atrialfibrillation.

Investigators have shown that atrial fibrillation can be eliminated withmultiple lines of ablative lesions placed in the right and left atria toemulate the surgical maze procedure. Failures of the ‘percutaneous maze’procedure have resulted primarily from incomplete lesion lines. MRIguidance should allow rapid confirmation of lesion line continuity andavoidance of unnecessary repetition of RF application where tissue hasalready been successfully ablated.

The MRI-guided catheter ablation system offers advantages in ablation ofischemic and idiopathic ventricular tachycardias, ectopic atrialtachycardias, atrial flutter, and atrial fibrillation. Unlike AV nodereentry and accessory pathway mediated tachycardia, these otherarrhythmias have lower ablation success rates and longer ablationprocedure durations, primarily due to difficulties in accurateactivation mapping or confirmation of lesion development withconventional equipment.

Procedure durations and risk of complications should thus be reducedsubstantially with the MRI-guided catheter ablation system.

A system and method for using magnetic resonance imaging to increase theaccuracy of electrophysiologic procedures is disclosed. The system inits preferred embodiment provides an invasive combined electrophysiologyand imaging antenna catheter which includes an RF antenna for receivingmagnetic resonance signals and diagnostic electrodes for receivingelectrical potentials. The combined electrophysiology and imagingantenna catheter is used in combination with a magnetic resonanceimaging scanner to guide and provide visualization duringelectrophysiologic diagnostic or therapeutic procedures. The system isparticularly applicable to catheter ablation of cardiac arrhythmias. Inembodiments which are useful for catheter ablation, the combinedelectrophysiology and imaging antenna catheter may further include anablation tip, and such embodiment may be used as an intracardiac deviceto both deliver energy to selected areas of tissue and visualize theresulting ablation lesions, thereby greatly simplifying production ofcontinuous linear lesions. The system further includes embodimentsuseful for guiding electrophysiologic diagnostic and therapeuticprocedures other than ablation. Imaging of ablation lesions may befurther enhanced by use of MR contrast agents. The antenna utilized inthe combined electrophysiology and imaging catheter for receiving MRsignals is preferably of the coaxial or “loopless” type. High-resolutionimages from the antenna may be combined with low-resolution images fromsurface coils of the MR scanner to produce a composite image. The systemfurther provides a system for eliminating the pickup of RF energy inwhich intracardiac wires are detuned by filtering so that they becomevery inefficient antennas. An RF filtering system is provided forsuppressing the MR imaging signal while not attenuating the RF ablativecurrent. Steering means may be provided for steering the invasivecatheter under MR guidance. Other ablative methods can be used such aslaser, ultrasound, and low temperatures.

Embodiments of the system and method disclosed herein include:

A method for performing an electrophysiological procedure, comprisingthe steps of: placing a subject in a main magnetic field; introducing anMR-compatible electrode catheter; acquiring a magnetic resonance signal;using magnetic resonance imaging to determine the location of saidMR-compatible electrode catheter; and, using said MR-compatibleelectrode catheter to acquire electrical signals indicative of anelectrophysiological state. The electrical signals may be indicative ofan electrophysiological state comprise intracardiac electrograms. TheMR-compatible electrode catheter may include a tip comprising gold. TheMR-compatible electrode catheter may comprise an MR-visible material.such as a metal. The method may include using a magnetic resonancecontrast agent to enhance images acquired in said step of using magneticresonance imaging to determine the location of said MR-compatibleelectrode catheter. The method may include acquiring a magneticresonance signal comprises the step of using a magnetic resonanceimaging antenna which is integral with said MR-compatible electrodecatheter.

A method for treating cardiac arrhythmias, comprising the steps of:placing a subject in a main magnetic field; introducing an ablationcatheter; and, using magnetic resonance imaging to visualize ablationlesions created using said ablation catheter. The method may includeusing a catheter antenna to receive magnetic resonance signals, whereinsaid catheter antenna comprises a loopless antenna or a loop antenna.The method may also include using a magnetic resonance contrast agent toenhance the visibility of said ablation lesions in MR images, whereinthe contrast agent is gadolinium-DTPA.

A system for performing ablation therapy, comprising:

ablation tip means for applying ablative energy to create ablationlesions; catheter means for inserting said diagnostic electrodes into aregion to be treated; magnetic resonance antenna means, integral withsaid catheter means, for receiving magnetic resonance signals; and,means for analyzing said received magnetic resonance signals and fordisplaying an image of said region to be treated, whereby said means forinserting can be guided to said region to be treated. The system forperforming ablation therapy may further comprise: a plurality ofdiagnostic electrodes; a loopless antenna; or a means for generating RFablation current; that includes a means for generating an RF magneticresonance imaging signal in a first frequency range; and a filter meansfor filtering said first frequency range from said RF ablation current.The antenna may receive induced magnetic resonance signals and be aninvasive catheter antenna, wherein the catheter antenna comprises aloopless or loop antenna. The means for generating an RF magneticresonance imaging signal may comprises means for generating a RFmagnetic resonance signal (64 MHz for 1.5 Tesla) and wherein said filtermeans may comprise means for filtering said resonance signal from saidablation signal. The filter means may comprise a low-pass filter, amulti-stage filter, a filter for filtering gradient-induced noise, or afilter for filtering gradient-induced noise comprises a series of activefilters.

A method for performing an electrophysiological procedure, comprisingthe steps of: placing a subject in a main magnetic field; introducing aninvasive imaging antenna; acquiring a first magnetic resonance imagefrom said invasive imaging antenna; acquiring a second magneticresonance image from a surface coil; combining said first and secondmagnetic resonance images to produce a composite image; and, using saidcomposite image to guide said electrophysiological procedure. The stepof using said composite image to guide said electrophysiologicalprocedure may comprise using said composite image to guide an ablationprocedure, such as to construct a three-dimensional map of areas in theheart that have undergone ablation or to construct a three-dimensionalrendering of the heart. The method may include storing saidthree-dimensional rendering into a texture map of an imaging volume.

A system for magnetic resonance imaging-guided catheter ablation,comprising: electrode means for receiving electrical signals indicativeof an electrophysiological state, said electrical signals being within afirst frequency range; means for generating an RFmagnetic-resonance-inducing signal; magnetic resonance antenna means forreceiving magnetic resonance signals from a region to be treated; filtermeans for filtering said first frequency range from said RFmagnetic-resonance-inducing signal.

A combined electrophysiology and imaging catheter, comprising: at leastone diagnostic electrode; catheter for inserting said diagnosticelectrode into a region to be studied; and, an invasive magneticresonance antenna, integral with said catheter, for receiving magneticresonance signals. The combined electrophysiology and imaging cathetermay further comprise: steering device for deflecting said catheter,whereby said catheter can be steered to said region to be studied, suchas by a steering wire, wherein said steering wire is of a titanium orother non-magnetic construction and is housed in a sheath. The steeringdevice may comprise a steering knob that is operable to move a steeringwire toward or away from a distal tip of said catheter. The combinedelectrophysiology and imaging catheter may further comprise: an ablationtip for applying ablative energy to a region to be treated and aflexible antenna whip at a distal portion of said catheter. The whip maybe coated with an insulating layer and comprise a tip which is formedinto a J-shape to prevent perforation. The catheter and all componentshoused therein may be fabricated of materials having low-magneticsusceptibility. The tip may be at least 4 millimeters in length,suitable for use in RF ablation procedures and fabricated from platinumor gold.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

We claim:
 1. A method for performing an electrophysiological procedureon a mammalian patient having a heart, comprising: positioning themammalian patient such that the heart is in a main magnetic field of anMRI scanner; introducing an MR-compatible catheter into the heart;acquiring a magnetic resonance image of the heart by applying static andgradient magnetic fields to the mammalian patient and sensing radiofrequency (RF) emissions from precession of protons in the heart excitedby the static and the gradient magnetic fields; using the magneticresonance image to position said MR-compatible catheter in the heartwhile continuing to acquire additional magnetic resonance images of theheart, establishing a conductive connection between the catheter and theheart while the catheter is in the static and gradient magnetic fields,and monitoring electrical signals in the heart acquired through theconductive connection between the catheter and the heart.
 2. The methodaccording to claim 1 further comprising filtering acquired electricalsignals to suppress electrical currents induced in the MR-compatiblecatheter due to the static and gradient magnetic fields.
 3. The methodaccording to claim 1 further comprising ablating the heart with theMR-compatible catheter to create ablation lesions in the heart andmagnetic resonance imaging the lesions.
 4. The method according to claim3 further comprising viewing an image of the ablation legions toidentify an area proximate to the ablation lesions and further ablatingthe area with the MR-compatible catheter.
 5. A method of performing amagnetic resonance-guided procedure comprising: placing a subject in amagnetic resonance scanner; acquiring magnetic resonance images of thesubject by applying static and gradient magnetic fields to the subjectand sensing radio frequency (RF) emissions from precession of protons inthe subject excited by the static and the gradient magnetic fields;identifying a target site in the subject using data about the subjectobtained from the magnetic resonance scanner; introducing into thepatient a magnetic resonance probe, wherein the magnetic resonance probeincludes (i) a catheter sheath, (ii) a center core within the cathetersheath and is configured to extend from the catheter sheath, wherein thecenter core is formed of a non-ferromagnetic conductive material and iscovered, at least partially, by a first insulating layer coating andwherein a distal end region of the center core forms a first pole of amagnetic resonance dipole antenna; (iii) a dielectric second insulatinglayer at least partially covering the catheter sheath, and (iv) aplurality of non-ferromagnetic conductive electrodes on an outer surfaceof the catheter sheath; advancing the magnetic resonance probe to thetarget site; and performing a procedure on the target site using themagnetic resonance probe in which electrical energy is conducted betweenthe conductive electrodes and the target site and while the magneticresonance scanner continues to acquire magnetic resonance images of thesubject.
 6. The method of claim 5, wherein the target site is located ina heart of the patient.
 7. The method of claim 5, wherein the magneticresonance probe is an RF ablation electrode, and the magneticresonance-guided procedure further comprises ablating heart tissue.